This invention relates to a magnetic bubble memory element. In particular, the present invention is concerned with a magnetic bubble memory element prepared by dividing the wafer which includes a plurality of island shaped magnetic bubble memory units (called memory unit hereinafter) and a plurality of separating regions intervening between the memory units above into separated memory units (magnetic bubble memory elements) and a method of manufacturing the same.
Generally, in a memory device making use of magnetic bubble memory elements, information "1" or "0" is discriminated by the presence of magnetic bubble, and memory operation in such a device is carried out by letting the magnetic bubble line in the closed loop circuit for transporting magnetic bubbles memorize information. In the device described above, it has been often experienced that normal memory operation is disturbed by generation of hard bubbles (abnormal magnetic bubbles) in the single crystalline magnetic thin film in the magnetic bubble memory element. This abnormally generated bubble is entirely different from the bubble which is made use of as a medium for memory. As a method of suppressing generation of hard bubbles, it is well known to form a thin film of a soft ferromagnetic material such as permalloy on the single crystalline magnetic thin film by deposition or other suitable way. The present invention is especially good for the magnetic bubble memory element which is provided with the hard bubble suppression film, so that the discussion will be developed in the following paragraphs in connection with the magnetic bubble memory element of that kind.
In the prior art method of manufacturing the magnetic bubble memory element of this class, the following steps have been taken which comprise forming a single crystalline magnetic thin film on the entire surface of a single crystalline substrate, forming a hard bubble suppression film on said thin film, forming a first insulating layer uniformly on said hard bubble suppression film and then forming a conductor pattern layer, a second insulating layer, a magnetic film pattern layer and a third insulating layer in order, thereby the wafer which includes a plurality of island shaped memory units being completed. In the process of completing the wafer, there is formed along the periphery of each of memory units a margin (called a separating region hereinafter) which has no concern with the function of the magnetic bubble memory element. By cutting the wafer along separating regions, memory units are separated from one another to become magnetic bubble memory elements.
According to the method above, however, the individual magnetic bubble memory element is prepared by dividing the wafer into pellets. Division of the wafer is usually performed by the process of scribing or dicing. This process, however, can not be proceeded without giving insulating films near the cut portion a lot of fine cracks which are caused by a physical force during the cutting work. The cracks are originated from the cutting portion in the separating region and usually extend to the inside of the memory unit. Since the suppression film is extending to the cutting portion, the cracks generated in the insulating film just on the suppression film invite serious problems, for example, oxidation of the suppression film which is caused by water vapour or oxygen in the air coming into the cracks. Same problem will be raised in the magnetic pattern layer and the conductor pattern layer when the cracks are generated in insulating films lying on those layers. By these phenomena, the function and reliability of the memory element are often fatally damaged.
In order to overcome the difficulty above, it has been tried to widen the width of separating region to the extent that cracks due to cutting stress can not reach the inside of the memory unit. This countermeasure, however, could not be essential, or rather it gave such an undesirable result that density of memory unit formation on the wafer is decreased to a large extent.